你的代码有一些问题
<?php
$title = $_POST['title'];
$link = $_POST['link'];
$description = $_POST['description'];
//$rss = new SimpleXMLElement($xmlstr); // need to have $xmlstr defined before you construct the simpleXML
//$rss->loadfile("feed.xml");
//easier to just load the file when creating your XML object
$rss = new SimpleXML("feed.xml",null,true) // url/path, options, is_url
$item = $rss->channel->addChild('item');
$item->addChild('title', $title);
$item->addChild('link', $link);
$item->addChild('description', $description);
//header("Location: /success.html");
//if you want to redirect you should put a timer on it and echo afterwards but by
//this time if something went wrong there will be output already sent,
//so you can't send more headers, i.e. the next line will cause an error
header('refresh: 4; URL=/success.html');
echo $rss->asXML(); // you may want to output to a file, rather than the client
// $rss->asXML('outfputfile.xml');
exit;
?>